In general, the spectrum of a VHF (very high frequency) or UHF (ultra high frequency) radio-frequency television signal lies between 48 MHz and 860 MHz. Within this range, a plurality of channels are covered and each channel is typically allocated with a bandwidth of 6˜8 MHz. In NTSC standard, a video carrier frequency positions at 1.25 MHz above the lower-margin of the bandwidth; a color carried frequency positions at 3.58 MHz higher than the video carrier frequency; while an audio carrier frequency positions at 4.5 MHz higher than the video carrier frequency. For example, Channel 2 has a bandwidth of 6 MHz ranged from 54 MHz to 60 MHz. Meanwhile, the video carrier frequency, color carrier frequency and audio carrier frequency position at 55.25 MHz, 58.83 MHz and 59.75 MHz, respectively.
When the radio-frequency television signal having the above-mentioned feature is received by an antenna or a cable and transmitted to the television tuner, the television tuner selects one or more channels accordingly, and converts the associated signal into an intermediate frequency (IF) signal or a base-band frequency signal which are subsequently processed by a demodulator.
In early ages, a conventional CAN tuner includes a mixer for directly down-converting the radio-frequency television signal into an intermediate frequency signal or a base-band frequency signal, as illustrated in FIG. 1. The radio-frequency television signal (I) including a plurality of channels multiplies a reference signal (II) with a reference frequency fLO in the mixer. In this way, a channel A with a frequency band positioned at the reference frequency fLO can be down-converted into a base-band frequency signal.
Applying a switching mixer, the reference signal is a square-wave signal having a 50% duty cycle. A plurality of harmonic frequencies, e.g. 3 fLO, 5 fLO, 7 fLO, etc., coming from the reference signal are also down-converted to the base-band frequency signal together with the reference frequency fLO. Channels B, C and D position at the harmonic 3 fLO, 5 fLO, 7 fLO, respectively. In other words, the associated signals of the channels A, B, C and D all appear in the base-band signal (III).
For readily identifying the desired channel A, the signal power of the channel A is supposed to be much higher, for example 30 dB higher than any co-channel interferer to maintain a desirable signal quality. Assuming on a condition that the signal power of either channel B, C or D is adversely 30 dB higher than that of the channel A, a component for providing at least the 60-dB harmonic rejection ratio is required to alleviate possible SNR (signal-to-noise ratio) degradation.
Conventionally, several RF (radio-frequency) tracking filters are utilized to reduce the harmonic power of the radio-frequency television signal prior to the entrance of the radio-frequency television signal into the mixer, in order to solve the above-mentioned harmonic problem.
However, such a tuner is generally bulky due to the incorporation of several discrete electronic devices such as the above-mentioned RF tracking filters, and thus is called as a CAN tuner. Moreover, the RF tracking filters require a high voltage varactor for band tuning. It is thus not only cost-ineffective but also hard to be integrated into a single-chip IC (integrated circuit) for reducing the module footprint.
With remarkable advances of semiconductor technologies to produce highly integrated silicon chips, a television tuner can also be manufactured as a single chip so as to reduce device size and cost. U.S. Pat. No. 5,737,035 discloses such a television tuner and is incorporated herein for reference.
A single-chip television tuner as suggested by U.S. Pat. No. 5,737,035 is illustrated in FIG. 2, wherein an up-down dual conversion tuner with two mixers is used. After a radio-frequency television signal is received by an antenna 402 (or a cable), it is first subjected to a cutoff frequency of, e.g. 900 MHz, an RF lowpass filter 404 to result in a frequency band below 900 MHz. Then the filtered radio-frequency television signal passes through a low-noise transconductance amplifier (LNTA) 406 so as to be amplified with a certain gain, e.g. 20 dB. Subsequently, the amplified radio-frequency television signal is mixed with a first reference signal in a first mixer 408 to output a first intermediate frequency signal. The first intermediate frequency signal is then mixed with a second reference signal in a second mixer 410 to output a second intermediate frequency signal so as to extract a desired channel with a specified carrier frequency.
The first mixer 408 is a subtractive mixer and the first reference signal is generated by a first local oscillator 411 to have an operating frequency tunable in the range between 1200 MHz and 2100 MHz. By adjusting the frequency of the first reference signal, the carrier frequency of a desired channel included in the resulting first intermediate frequency signal can be obtained as 1200 MHz. On the other hand, the second mixer 410 is an image rejection mixer and the second reference signal is generated by a second local oscillator 412 to have an operating frequency of 1180 MHz. With the mixing of the second reference signal, the carrier frequency of the desired channel included in the resulting second intermediate frequency signal can be obtained at 20 MHz, i.e. (1200-1180) MHz. The first mixer 408 up-converts the carrier frequency of the desired channel to 1200 MHz to minimize harmonic effects, and then the second mixer 410 down-converts the carrier frequency of the desired channel to 20 MHz.
This tuner rejects signal energy in the first intermediate frequency signal that is in the area of 1160 MHz, which would also yield |1160-1180|=20 MHz. FIG. 3 illustrates another up-down dual conversion tuner similar to the tuner of FIG. 2 but further including an IF filter 409 between the mixers 408 and 410 to reject the image frequency component 1160 MHz from the first intermediate frequency signal.
The above-mentioned single-chip television tuners use a reference signal with a tunable frequency and another reference signal with a fixed frequency to mix the radio-frequency television signal to obtain the carrier frequency of the desired channel. In order to adjust the frequency of the first reference signal in a relatively wide range, a plurality of voltage controlled oscillators (VCOs) are needed and that results in higher circuit complexity and chip size.